The mass spectrometer is an analyzer capable of measuring the abundance for every mass number of sample components, and has such large characteristic as extremely high sensitivity (low detection limit) as compared with other analyzers. One of elements realizing this is a unit (an ion detection unit) detecting a mass-selected ion, which includes a secondary electron multiplier (SEM), a deflection board, a collector etc. The SEM has electrodes of around 20 stages, to which a voltage is applied in sequence from the first-stage electrode (D1) to the final-stage electrode with a potential difference of around 100 V to 200 V. In many cases, respective voltages are supplied by dividing an applied voltage to the first-stage electrode D1 by a resistance inside the SEM. To the electrode D1, −1.5 kV to 3 kV is applied, against which mass-selected ions collide with an energy corresponding to the potential thereof. When 2 kV is applied to the electrode D1, a collisional energy is approximately 2 keV, and, when the energy is at such level, electrons (secondary electrons) are generated on the surface of the electrode D1. That is, on the electrode D1, ion/electron conversion is performed. The yield (efficiency) is around 0.1 to 0.2, and is influenced little by the material and state of the surface of the electrode D1. However, it depends on the collision energy, and is generally proportional to the collision energy E. Therefore, the energy of the ion is important for the ion/electron conversion.
The electron generated from the electrode D1 is pulled in the second stage electrode (D2) having a potential higher (in the plus direction) by around 100 V than that of the electrode D1, and collides with an energy of around 100 eV corresponding to the potential difference. The yield of the generation of secondary electrons by an electron collision is very high, and, in the energy at such level, is around 1.5 to 2.0 in an appropriate surface state. Accordingly, the amplification of the electron is realized here. After that, the electron amplification is performed in the same manner on the third-stage electrode D3 and the fourth-stage electrode D4. On the final-stage electrode, the amplification of 5 digits to 6 digits can be performed (see Patent Documents 1 to 3, Non-patent Document 1). The SEM capable of performing such extremely high amplification is an indispensable unit for the mass spectrometry.
However, to the ion detection unit (the ion detector), not only ions to be detected that are mass-selected and become signal, but also light causing noise arrives. On the electrode D1, the light is also converted to an electron, and, therefore, the light entering the electrode D1 being the first-stage electrode is amplified on the electrode D2 and subsequent electrodes, as is the case with the signal. Consequently, no matter how an amplification factor is made higher, S/N (a signal/noise ratio) is not improved to lead to meaningless. For a mass spectrometer, measurement of sample components having concentration difference of 5 to 6 digits is expected, and, for this, S/N of 6 digits is necessary. Therefore, how suppress the noise dominates decisively the performance.
In order to perform mass fractionation, first, a neutral molecule to be measured needs to be made into an ion (be ionized). A representative ionization method is called an electron ionization, in which a thermal electron of around 70 eV at an ion source is collided against a neutral molecule and an outer-shell electron is flicked to give an ion (in addition, there are several methods such as using a plasma in place of the thermal electron). However, many molecules not ionized but exited are generated in this course, which, after a little, emit light (an electromagnetic wave of around several to 50 eV: vacuum ultraviolet light) and stabilize. The vacuum ultraviolet light has no charge, and, therefore, reaches the ion detection unit without being fractionated by the mass spectrometer. If the light is irradiated as it is to the first-stage electrode D1 of the SEM, an electron (also referred to as a photoelectron because it is caused by light) is emitted. The efficiency thereof is, although strongly dependent on the surface state, approximately 0.1 (0.01 to 0.2). That is, electrons are generated with efficiency nearly equal to that of ions being the original signal.
Moreover, such probability is known that light causing the noise is also emitted from the mass spectrometer. In the most representative quadrupole mass spectrometer, against a quadrupole pole having a negative high voltage, an ion collides to generate light (an electromagnetic wave of around 50 eV to 2 keV: soft X ray) having an energy higher than that of vacuum ultraviolet light. FIG. 9 shows an aforementioned situation.
FIG. 9 is a drawing schematically showing a conventional mass spectrometer.
In FIG. 9, a mass spectrometer 1 disposed in a vacuum vessel 1a includes an ion source 2, a mass fractionation part 3, and an ion detection unit 4.
The ion source 2 has a filament 5, uses thermal electrons 6 generated by the filament to ionize neutral molecules including a molecule of a measurement object, and introduces generated ions into the mass fractionation part 3.
The mass spectrometer 3 has quadrupole electrodes 7 including four column-shaped electrodes. Among quadrupole electrodes 7, electrode sets facing each other are electrically coupled and, to respective electrode sets, a direct-current voltage and a high-frequency alternating-current voltage are applied to cause only ions having a mass number corresponding to respective voltages, frequencies etc. to pass in the long axis direction of the quadrupole electrode 7.
The ion detection unit 4 has a deflection board 8 functioning as an electrode for curving the trajectory of ions, and a secondary electron multiplier (SEM) 9. In FIG. 9, the secondary electron multiplier 9 has electrodes D1 to D20 of 20 stages (20 pieces), and a collector 10. To the ion introduction part of the ion detection unit 4 at a former stage in the deflection board 8, a mass aperture board 11 in which an aperture is formed is provided. The mass aperture board 11 is set to have a predetermined potential, and usually has the ground potential (the earth potential, 0 V).
In such constitution, when ions generated in the ion source 2 enter the mass fractionation part 3, an ion 13 having a desired mass number passes through the mass fractionation part 3, and the ion 13 enters the ion detection unit 4. The ion 13 having entered the ion detection unit 4 changes the trajectory thereof toward the electrode D1 side by the action of the deflection board 8. When the ion 13 enters the electrode D1, on the electrode D1, an electron is generated by ion/electron conversion. When the electron enters the electrode D2, on the electrode D2, secondary electrons 12 are generated, and the secondary electrons 12 are amplified sequentially on subsequent electrodes D3 to D20 to enter the collector 10. The collector 10 outputs a signal 15 in accordance with entered secondary electrons.
In such mass spectrometry, as described above, in addition to ions, vacuum ultraviolet rays 15 may enter the mass fractionation part 3 from the ion source 2. Moreover, by the collision of an ion 16 against the quadrupole electrode 7, soft X rays 17 may be generated.
The vacuum ultraviolet light and the soft X ray thus generated cause the noise for the mass spectrometer, and are called “stray light.” Since the stray light dominates largely the basic performance of the mass spectrometer, the ion detection unit is devised so as to detect an original ion alone and not to detect the stray light as far as possible, but sufficient performance is not necessarily given.    [Patent Document 1] Japanese Patent Laid-Open No. 2002-329474    [Patent Document 2] Japanese Patent Laid-Open No. 10-188878    [Patent Document 3] Japanese Patent Laid-Open No. 2001-351565    [Non-patent Document 1] Ohmura Takayuki, Yamaguchi Haruhisa, “Detector of mass spectrometer-secondary electron multiplier,” J. Vac. Soc. Jpn., Vacuum Vol. 50, No. 4, P 258-263 (2007)